Electrical lighting devices, including incandescent, fluorescent, high intensity discharge and halogen lamps, as well as cathode ray tubes, usually comprise a hermetically sealed glass envelope. Electrodes, incandescent filament, exhaust tubes and other components are hermetically sealed and affixed to the glass envelope. The electrodes and the incandescent filaments are generally housed within the glass envelope. The lead wires of the electrodes must be hermetically sealed with the glass envelope. Beaded leads are traditionally used in these many lighting devices to effect the glass-to-metal seals. FIG. 1 shows schematically the design of a traditional beaded leads, which consists of three pieces: an inner nickel or tungsten lead 13, an outer stranded nickel lead 11, and an intermediate tungsten lead 15. A glass bead 17 is hermetically sealed over the middle tungsten lead. For borosilicate lamp applications, the bead is typically made of borosilicate glass as well. Traditionally, flame burner technology is used in sealing the bead to the glass envelope. Burners provide the necessary heat to soften and fuse the two glass components together to form a hermetic seal. Having glass-to-glass contact ensures that the electrical feed-through is fully hermetic within the lighting device.
A typical fluorescent lamp comprises a glass envelope having an enclosed discharge channel. Inside the discharge channel are placed electrodes, mercury, getters, phosphor coatings and inert gases. Inert gases and/or mercury vapor sealed inside the channel, excited by electrical energy, emit ultraviolet radiation together with a small amount of visible light. Phosphor particles, usually sized on the micro level, covert the ultraviolet radiation into visible light via the mechanism called fluorescence. As in other lighting applications, the exhaust tube is sealed outside of the glass lamp envelope. Recently, a new generation of fluorescent lamp has been developed especially for use as the backlighting unit for information displays, such as the LCD display and other displays requiring an illumination source. These lamps feature a flat geometry having a convoluted discharge channel enclosed inside. To achieve good lighting efficiency and appealing color, rare-earth tri-phosphors are typically applied onto the internal wall of the discharge channel on these lamp envelopes. As in other fluorescent light devices, getters, electrodes, mercury and inert gases as well as exhaust tubes are to be hermetically sealed with the glass envelope. A method of producing such flat panel lamp envelope is disclosed in U.S. Pat. No. 6,301,932. The glass envelope produced in accordance with this method has a once-piece design, viz., the faceplate and the backplate are sealed together to form an integral lamp body having a convoluted channel. To meet the requirements of many applications, especially those of the portable devices, such as a notebook computer, a handheld computer, and the like, light-weight flat panel backlight unit has been produced. These lamp envelopes may have a very thin substrate with a wall thickness often even below 1 mm.
For these flat panel lamp envelopes, especially the one-piece lamp envelope produced in U.S. Pat. No. 6,301,932, electrodes, getters and exhaust tubes are typically sealed with the lamp envelope after the phosphor coating has been applied. Hermetic sealing the electrodes, exhaust tubes and other components to the lamp envelope has been proven to be a challenge. Flame sealing technology suitable for traditional lighting devices has proven to be difficult, if not impossible, to be directly used for these applications in the conventional way. First, the excessive heat generated by the burner flame will soften the glass envelope, causing the thin glass walls to sag and leading to deformation. The serpentine channel geometry requires sealing on a flat surface often of glass thickness less than 1 mm. Though possible, flame working glass components to a flat surface typically results in a deformed seal area, subsequently affecting the overall thickness of the lamp and mechanical strength. Second, these lamp envelopes with large area and complex geometry are very sensitive to temperature differentials. The temperature differentials involved in flame sealing can easily cause them to crack. Third, the phosphor coatings, getter and electrodes are sensitive to high temperature. Exposure of the phosphor coating to a temperature over 600° C. will greatly diminish its functionality, lowering light output and subsequent life. Getters, when subjected to high temperature before the channel is evacuated and hermetically sealed, may be activated prematurely, react with the atmosphere in the channel, and thus lose their intended functions. The electrode bells are usually coated with a layer of special emission coating material on its outer surface. When heated to a temperature over 600° C., the coatings will be destroyed or negatively affected as well. Oftentimes, using direct flame sealing technology in a conventional manner cannot avoid heating the phosphors, getters and electrodes, which are located usually not far from the sealing area. Some natural outcomes, therefore, for example, are destroyed or deteriorated lamp components, lamp dysfunction or shortened lamp life, and reduced lamp brightness uniformity. Fourth, in many lamp applications, the holes to be sealed are much larger than the solder glass bead on a beaded lead wire, making it difficult, if not impossible, to directly seal the beaded lead wire to the glass envelope using conventional flame sealing technology. Finally, using a burner flame can introduce unwanted impurities into the lamp discharge channel such as hydrocarbons, which are detrimental to the lamp brightness, brightness uniformity and lamp life.
Therefore, there is a genuine need of a new approach for sealing the flat panel lamps in place of the conventional direct flame sealing technology, where localized heating can be used to effect a hermetic sealing and affixing lamp components, such as electrode leads and tubulations, without affecting the crucial and sensitive lamp components.
Solder glasses have been used to achieve glass-to-glass or glass-to-metal sealing. Solder glasses can be vitreous or devitrifying. Vitreous solder glasses maintain a glass state after sealing. They are thermoplastic materials which melt and flow at the same temperatures each time they are melted. Devitrifying solder glasses, commonly referred to as frits, are thermosetting, meaning they are no longer in glassy state after sealing, but contains both glassy and crystalline phases. Because of their thermosetting properties, devitrifying solder glasses have many advantages and preferred in many applications against the vitreous ones. Once crystallized, devitrifying solder glasses have a higher modulus of rupture of about 4.2–5.6×10−6 kg·m−2 compared with about 2.1–3.5×10−6 mg·m−2 for its vitreous precursors and counterparts. Moreover, the softening point of the devitrified frits is essentially increased to above the initial softening point of their vitreous precursors. The overall effect would be a seal stronger and more stable at elevated temperatures.
Solder glasses enable the use of localized heating to effect a hermetic seal. Besides, solder glass can be used to fill in the gaps between a component, such as the bead of a beaded lead wire and tubulations such as the exhaust tube, and the periphery of a larger hole in which the component is to be hermetically sealed. Therefore, they are attractive in sealing the flat panel lamps.
Because an electrical lamp generates heat during its operation, it is desirable to use a low-expansion glass to manufacture glass envelopes. Borosilicate glasses having a coefficient of thermal expansion (CTE) in the range of 30–45×10−7° C.−1 from 0 to 300° C. have been used for lighting devices, including flat panel lamp envelopes. However, it has proven to be challenging to find the suitable solder glass and sealing method for sealing the lamp components with the borosilicate lamp envelope.
The present invention, by providing a process for sealing and affixing a component assembly comprising a solder glass preform and the component to be sealed by using induction heating, addresses the concerns outlined above.